1. Field of the Invention
The present invention relates to a technology for forming an image, and more particularly, to a technology for forming a latent image by multibeam exposure.
2. Description of the Related Art
With the functional improvement of image forming apparatuses, proportion of forming images represented by prints per minute (PPM) is increasing. Some image forming apparatuses capable of high-speed and high-quality image forming perform multibeam exposure by using a surface emitting laser such as a vertical-cavity surface emitting laser (VCSEL). Moreover, from the viewpoint of resource saving, more and more duplex-printing models have been provided.
Reflecting such situations, most manufactures try to reduce time between first printing on a front surface of a recording medium and second printing on a back surface of the same recording medium to develop an improved duplex-printing image forming apparatus. There appears a high-speed duplex-printing model that performs both the first printing and the second printing within 10 seconds. However, coupled with size-reduction of the image forming apparatuses, those duplex-printing models have difficulty in cooling. As time between the first printing and the second printing is reduced, the recording medium has to be attached to a high-temperature fixing unit for a longer time. In other words, the recording medium is hard to be cooled down because it is in a high-temperature situation.
When images are printed on both surfaces of the recording medium, for example, a paper sheet with about 80 micrometers in thick in such a difficult-to-cooling situation, a size of an image printed on the back surface is enlarged/reduced than a size of an image printed on the front surface by from 0.2% to 0.4% due to the fluctuation in temperature and humidity.
To solve the above problems, Japanese Patent No. 3373266 discloses a conventional technology for reducing/enlarging image data with respect to the sub-scanning direction by deleting/adding data from/to the image data with respect to the sub-scanning direction.
Although the conventional technology can suppress the size difference, it may form a defective size-adjusted image. The possibility of forming a defective size-adjusted image increases as the quality of image increases. For example, if a size of an image having a plurality of solid straight lines each spaced with five lines is adjusted by adding/deleting a line, a size-adjusted image with a remarkable defect such as uneven density or moiré patterns will be formed.